Until the present invention the only light pen systems that were usable for high resolution and high accuracy graphics work were those implemented with vector writing CRT's. Raster scan CRT systems were limited to low accuracy picking, such as in making menu selections. It would be desirable if the performance of a vector writing light pen system could be obtained in a raster scan system, with its relatively lower cost and level of drive circuit complexity.
It is conventional to track a light pen with a cursor. However, it has heretofore been infeasible to accurately and with high resolution track a light pen in a raster scan system because the comparatively long decay time of the CRT phosphor effectively means that there can only be one "hit" (detected output from the light pen) per scan line. In a raster scan system there might be little or no temporal separation between pixels that are user data and pixels that are part of the cursor. Thus, an initial hit caused by user data in a scan line also containing a potential hit for the cursor "absorbs" or prevents that potential hit on the cursor. Without accurate hits from the cursor the system cannot accurately track the light pen with that cursor, nor can the user accurately pick locations upon the screen.
A preferred embodiment of the present invention solves this problem by maintaining a small region of interlacing around the cursor. Within the region of interlacing the cursor is displayed in full each frame, while the user's data is divided into even and odd scan lines. Within the region of interlacing user data on odd scan lines is blanked every other frame, while user data on even numbered scan lines is blanked during the intervening frames. During the blanking of odd numbered scan lines the only hit on an odd numbered scan line will be one corresponding to the cursor, with a similar correspondence for even numbered scan lines during the intervening frames. When taking hits for odd numbered scan lines during a frame any hits on even numbered scan lines are suppressed, and vice versa. Thus, each frame can provide truthful cursor hit information from which can be calculated the relative displacement between the center of the cursor and the center of the field of view for the light pen, so that the cursor may be made to track the movements of the pen.
In conjunction with the interlacing technique mentioned above a threshold shifting technique raises the minimum intensity threshold needed for light reaching the pen to create a hit. The shifting to a higher level is done when a hit is expected from a horizontal component of the tracking cursor. This counteracts the build-up of light within the pen's field of view whenever a horizontal line is being written within that field of view. That allows the accurate measurment of where the left-hand edge of the horizontal component of the cursor intersects the field of view, even though there may be a considerable length of horizontal line to the left of the field of view (assuming a left-to-right direction of scanning).
The desirable result produced by the threshold shifting technique is potentially adversely affected by changes in display intensity and by changes in the distance from the light pen to the faceplate of the CRT. These adverse effects are avoided by focusing the light pen optics at infinity and by providing it with a divergent viewing region whose area increases as the distance from the light pen to the CRT increases. Such a light pen has a family of optical attenuation curves that are generally sinusoidal in shape. That, in turn, allows certain single dot type and horizontal line type fields of view to vary together in simultaneous proportion as functions of display intensity and pen-to-screen distance. That simultaneous proportional variation is highly desirable in a system that performs threshold shifting to obtain accurate measurements upon horizontal lines.